There are a number of different types of handovers that are used to transfer a user equipment (UE), such as a cellular telephone, from one base station to another. For base stations, such as Node Bs or evolved Node Bs (eNBs), that serve multiple cells (also called sectors), handovers occur between cells of the base stations. Typical terminology used includes soft handovers (using a make-before-break technique) and hard handovers (using a break-before-make technique). Soft handover versus hard handover concerns whether the UE connects to a single base station (hard handover) or multiple base stations (soft handover) at the time of the handover. Consequently, a soft handover is a handover that occurs between different base stations and where radio links are added and abandoned in such a manner that the user equipment always maintains at least one radio link to the radio access network. Thus, a connection is made with the new, target cell prior to a break from the original, source cell. It should be noted that a soft handover is always, an intra-frequency handover. In a hard handover, the UE breaks connection with the source cell prior to transitioning to and receiving from a target cell.
Further terminology includes backward and forward handovers. The terms “backward” or “forward” describe whether a handover (HO) command originates from source cell (backward HO) or target cell (or UE) (forward HO).
This terminology may also be combined. For instance, UMTS (universal mobile telecommunications system) uses backward, soft handover. A discussion of soft handovers in UTRAN (UMTS terrestrial radio access network) is given in 3GPP TS 25.331, v7.3.0, section 8.3. E-UTRAN (evolved UMTS terrestrial radio access network), also called 3.9G (generation 3.9) or LTE (long term evolution), currently uses backward, hard handover.
E-UTRAN is a transmission system based on packet data. This system supports intra-frequency, inter-frequency and intra-RAT (radio access technology) HO. It has been agreed that LTE will use a break-before-make, backward handover. Soft handovers are not expected to be used for LTE at the current time.
Concerning inter-frequency HOs, an intra-frequency handover is triggered when a UE is in the cell edge of current serving cell. By contrast, softer handover (e.g., intra-eNB) is a vendor implementation issue. A softer HO is a handover occurring between cells (i.e., sectors) only at the source cell. A softer HO is a make-before-break handover technique, and is an intra-frequency handover which occurs between cells (i.e., sectors) within one base station and where the radio links are added and abandoned in such manner that the user equipment always maintains at least two radio links to the base station.
In a frequency reuse-1 system (meaning that the entire allotted frequency spectrum is used) such as LTE, a UE may experience strong interference and low SINR (signal to noise and interference ratio) (e.g., less than −5 dB) in DL (downlink) when the UE is in the cell edge. Given such low SINR, the BLER (block error rate) of received data after ARQ (automatic repeat-request) and HARQ (hybrid ARQ) processes tends to be very low in some scenarios (e.g., UE crossing the corners with certain vehicular speeds such as greater than 30 km/hr, kilometers per hour). This raises concerns as to whether HO execution signaling (e.g., mainly HO command in DL, downlink) can be reliably communicated between eNBs and UEs. For instance, see details in the contributions R2-063281, R2-070213. In other words, this raises the concern whether there is a coverage problem in terms of HO execution signaling in LTE.
It is believed that the rate of failure of HO commands may be very high in some worse case scenarios, such as high mobility (UE speed >30 km/hour), one receiver (e.g., UE) antenna, and fully loaded scenarios. If the transmission (and reception) of the HO command fails, the UE will move to the idle state, and start cell reselection. The whole procedure due to the failure of HO command will cause a long (e.g., >500 milliseconds) interruption time. This is not a tolerable value for an LTE system.
As a result, it is necessary to improve the reliability of HO command signaling and other command signaling.